And it would not require extra resources. For Dark Souls, reducing the damage taken by half for "easy" and by half again for "way too easy" would probably suffice.

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Right this is the point it would not be difficulty to implement and is why these kinds of things are so easy to mod with a program like cheat engine, beside that things like subtitles, custom controls and difficulty setting should be standard game features. If for no other reason than to help disabled gamers, which people keep seeming to ignore.

An easy mode(or perhaps very easy mode) might be the only way some people will be able to play the game, as they physically won't be able to otherwise and for them the game might still be very hard, but at least doable just like it is for normal gamers.